Brake piston

ABSTRACT

The present invention relates to a brake piston with reduced transmission of heat from the disk to the brake fluid.  
     In particular, the present invention relates to a disk-brake piston ( 7 ) characterized in that it comprises a cup ( 12 ) having a skirt ( 14 ) and a head portion ( 15 ), and a tubular insert ( 13 ) housed in the cup, a space for cooling air being defined between the outer surface of the tubular insert ( 13 ) and the inner surface of the skirt ( 14 ) of the cup ( 12 ).

The present invention relates to a brake piston with reducedtransmission of heat from the disk to the brake fluid.

As is widely known, during braking, any brake is subject, as a whole, toconsiderable heating owing to the dissipation of the vehicle's kineticenergy to an extent which is limited to the proportion due to the wheelwith which the brake is associated. In disk brakes, this kinetic energy,a large part of which is converted into thermal energy because of thenecessary friction during the braking operation, is stored mainly by thedisk, from which it is then dissipated into the surrounding environment.However, an appreciable portion of the heat is also transmitted to thepads, from which it is transferred, by conduction, to the piston andfrom there, again by conduction, to the brake fluid.

Although, in the event of heavy or prolonged braking or in the event ofrepeated braking, it is not generally possible to eliminate an increasein the temperature of the brake fluid, it is essential that the brakefluid should not reach boiling point, since the braking effect would becompletely eliminated by the formation of vapour in the braking system.

The problem of the transfer of heat from the pads to the pistons and tothe brake fluid is experienced in particular in applications for sportscars, or even racing cars, and to an even greater extent with the use ofcarbon pads and disks which require relatively high temperatures toensure optimal operating characteristics.

The problem underlying the present invention is therefore to preventoverheating of the brake fluid, even with heavy and prolonged use of thebrake.

This problem is solved by a disk brake and by a piston as defined in theappended claims.

Further characteristics and advantages of the disk brake of the presentinvention will become clearer from the following description of somepreferred embodiments thereof, given by way of non-limiting example withreference to the following drawings:

FIG. 1 shows, in section, a disk brake according to the presentinvention,

FIG. 2 shows, in section, the piston of the disk brake according to thepresent invention,

FIG. 3 is an exploded perspective view of the piston of FIG. 2,

FIG. 4 shows the piston of FIG. 3, sectioned in a diametral plane,

FIG. 5 shows, in section, a second embodiment of the piston of the diskbrake according to the present invention,

FIG. 6 is an exploded perspective view of the piston of FIG. 5, and

FIG. 7 shows the piston of FIG. 6, sectioned in a diametral plane.

The disk brake according to the present invention will now be describedwith reference to the drawings.

The disk brake, generally indicated 1, comprises a caliper 2 arrangedastride a disk 3 (shown only partially in FIG. 1), and a pair of pads 4a, 4 b on which respective pistons 7 act.

The pads 4 a, 4 b are made of material with a high coefficient offriction (for example, carbon pads) and are intended to engage thebraking surfaces of the disk 3 during braking. Alternatively, each padis composed of a metal plate facing the piston 7, with which it is incontact during braking, and of a lining facing the disk and made ofmaterial with a high coefficient of friction.

With so-called floating calipers (not shown in the drawings), the brakecomprises a single piston which acts on one of the two pads.

The piston according to the present invention should be consideredapplicable to both of the above-described disk-brake types.

The piston 7 is housed for sliding in a respective cylinder 8 which isopen on the side facing the plate and into which the brake fluid comingfrom the hydraulic equipment of the braking system is admitted through aduct 9. A sealing ring 10 ensures the leaktightness of the chamber 11defined between the inner walls of the cylinder 8 and the outer walls ofthe piston 7.

The piston 7 comprises a cup 12 and an insert 13. The cup 12 has ahollow, substantially cylindrical shape, formed by a cylindrical skirt14 and by a head portion 15 which closes the skirt at one end. Theoutside diameter of the piston 7 substantially corresponds to the insidediameter of the cylinder 8 so as to permit axial sliding of the piston.The head portion 15 has a projecting peripheral rim 16 and a centralcylindrical element 17 which also projects, a recessed annular surfacebeing defined between the rim 16 and the central element 17.

As shown best in FIG. 4, an annular rib 18 with slightly inclined sidesand with a rounded ridge profile is defined on the inner surface of theskirt 14, in the vicinity of the connecting region between the skirt 14and the head portion 15.

According to a preferred embodiment of the present invention, thethickness of the skirt 14 is less than the thickness of the head portion15; most preferably, the thickness of the skirt 14 is approximately halfof the thickness of the head portion 15.

The insert 13 is substantially tubular with an outside diameter smallerthan the inside diameter of the cup 12 so that an annular space iscreated between the two surfaces. The insert 13 terminates in anoutwardly-projecting flange 19 at one end and has a length such that,once the insert 13 is inserted in the cup 12 in abutment with the innersurface of the head portion 15, the flange 19 is arranged outside thecup 12 and, radially, substantially at the level of the outer surface ofthe skirt 14.

A ring of teeth 20, constituting radial spacing means between the insert13 and the skirt 14, is disposed on the outer surface of the insert 13in the vicinity of the flange 19 but in a position such as to be able toengage and slide on the inner surface of the skirt 14 of the cup 12 whenthe insert is inserted in the cup. The function of the ring of teeth 20,in addition to that of keeping the insert 13 spaced from the skirt 14,is to centre the insert. At the same time, the presence of teeth ratherthan, for example, a continuous rib, also permits improved aircirculation to the exterior in the direction of the arrows in FIG. 2.

In the vicinity of its end remote from the flange 19, the insert 13 hasa thicker portion in which an annular groove 21 is formed externally.The groove 21 comprises an outer wall, closer to the end of the insertwhich is to engage in abutment with the head portion 15 of the cup 12,and a facing, inner wall. The inner wall of the groove 21 is constitutedby an annular projection 22 having a substantially rounded ridgeprofile. The annular projection 22 is positioned in a manner such that,when the insert 13 is inserted in the cup 12 in abutment with the innersurface of the head portion 15, the annular projection is aligned withthe annular rib 18 of the skirt.

The outer edge of the thicker portion of the insert 13 is chamfered toprevent it from interfering with the inner surface of the connectingregion between the skirt 14 and the head portion 15.

A plurality of through-holes 23 is formed in the insert 13, between theannular projection 22 and the ring of teeth 20, the holes being ofdimensions such as to permit an efficient circulation of air between theinterior of the insert 13 and the space between the insert and the skirt14.

The piston 7 also comprises a resilient ring 24 made of resilient metalor constituted by a split ring. The diameter of the resilient ring 24 issuch that it can be fitted in the annular groove 21—which constitutesits seat—so as to project radially farther outwards than the annularprojection 22.

As shown in FIG. 2, the resilient ring 24 is intended to be housed inthe annular groove 21, interfering with the inner surface of the skirt14 of the cup 12 in the region disposed between the annular rib 18 andthe region of connection with the head portion 15, and hence downstreamof the rib 18, with reference to the direction of insertion of theinsert 13 in the cup 12.

In a second embodiment of the piston 7, shown in FIGS. 5, 6 and 7, theinsert 13 comprises a tubular connecting portion 25 between the ring ofteeth 20 and the flange 19. This tubular connecting portion 25, whichhas the same diameter as the tubular insert 13, also has a plurality ofthrough-holes 23′ with the same function as the holes 23 describedabove.

The piston 7 is assembled by the insertion of the resilient ring 24 inthe seat 21 and the subsequent introduction of the insert 13 into thecup 12, in abutment with the inner surface of the head portion 15thereof. The resilient ring 24, which is deformed by interference withthe annular rib 18 during insertion, then expands downstream thereof,constituting a means for snap-retaining the tubular insert 13 inside thecup 12. Since the material of which the resilient ring 24 is made ismetal, the force to be imparted to the insert in order to remove it fromthe cup is considerable, requiring deformation of the resilient ring. Itis thus clear that the tubular insert 13 is restrained firmly inside thecup 12, keeping the piston firmly assembled.

The piston 7 is then inserted in the brake cylinder 8 with the headportion 15 facing the base of the cylinder.

The advantages of the disk-brake piston according to the presentinvention are clear from the foregoing description.

In the first place, the presence of a space between the tubular insert13 and the cup 12 permits the presence of a cushion of cooling airbetween the two surfaces, which constitutes a first insulation andheat-dissipation means.

In the second place, the fact that the tubular insert 13 projects fromthe cup 12 means that the cup does not come into contact with the brakepad, thus minimizing the direct transfer of heat from the pad to thebrake fluid by conduction. In fact, the sole opportunity for thermalconduction is between the pad and the tubular insert 13 which in turncan transfer only a limited proportion of the heat to the cup 12 byconduction, by virtue of the fact that the contact surface between theinsert and the cup is limited to the ring of teeth 20 and to the endsurface of the insert which is in abutment with the head portion 15 ofthe cup 12. Since it is the outer surface of the cup which is directlyin contact with the brake fluid, the arrangement described cansubstantially limit heating of the brake fluid.

Moreover, the provision of a ring of teeth 20 rather than a continuousprojection favours replacement of the cushion of air which is present inthe space between the tubular insert 13 and the cup 12, promoting thedissipation of the heat which tends to accumulate inside the piston.

Finally, the presence of the through-holes 23 in the body of the tubularinsert 13 permits a circulation of cooling air from the hollow interiorof the insert to the space between the insert and the cup and, fromthere, towards the exterior, as indicated by the arrows of FIG. 2. Inthis case, the dissipation of the accumulated heat is optimal.

The fact that the tubular insert can be inserted in the cup andcomprises snap-retaining means has two clear advantages. In the firstplace—by virtue of the presence of the metal resilient ring 24—theretaining means described ensure firm and stable assembly of the piston.Moreover, the absence of welding between the cup and the tubular insertallows the insert to be removed—after the application of sufficientforce—whenever it is necessary to disassemble the piston for maintenanceor replacement operations.

The embodiment shown in FIGS. 5, 6 and 7 permits even more efficientheat dissipation since the greater axial length of the insert permits alarger spacing between the pad and the brake-fluid chamber. Moreover,the possibility of providing a second series of holes 23′ tends tomaximize the circulation of air inside the piston and to minimize thecross-section for thermal conduction.

Clearly, only some specific embodiments of the disk-brake pistonaccording to the present invention have been described and a personskilled in the art will be able to apply thereto all modificationsnecessary to adapt them to particular applications without, however,departing from the scope of protection of the present invention.

For example, the ring of teeth 20 may be replaced by a continuous rib orby other means for spacing the surface of the insert from the innersurface of the skirt 14 of the cup 12.

The snap-retaining means may alternatively be constituted by other knownsnap means, for example, by teeth disposed on the surface of the insertand cooperating with a groove formed in the inner surface of the skirt14 of the cup 12.

The through-holes 23, 23′ may differ in number and shape, for example,they may be in the form of longitudinal or transverse slots.

1. A disk-brake piston, comprising a cup having a skirt and a headportion, and a hollow tubular insert housed in the cup, wherein a spacefor cooling air is defined between an outer surface of the tubularinsert and an inner surface of the skirt of the cup.
 2. A pistonaccording to claim 1 in which the tubular insert comprisessnap-retaining means for retaining the insert in the cup, the tubularinsert being removable from the cup after the application of an adequateremoval force.
 3. A piston according to claim 2 in which thesnap-retaining means comprise a resilient ring housed in an annulargroove formed in the outer surface of the tubular insert and cooperatingwith an annular rib on the inner surface of the cup.
 4. A pistonaccording to claim 3 in which the resilient ring is made of metal or isa metal split ring.
 5. A piston according to claim 1, in which thetubular insert comprises means for the radial spacing of the surface ofthe insert from the skirt of the cup.
 6. A piston according to claim 5in which the radial spacing means are a ring of teeth which define ductsfor the circulation of air between the space between the insert and theskirt of the cup and the exterior of the piston.
 7. A piston accordingto claim 1, the tubular insert projecting axially from the cup.
 8. Apiston according to claim 1, the tubular insert comprising a pluralityof through-holes for putting the hollow interior of the insert intocommunication with the space between the insert and the skirt of thecup.
 9. A piston according to claim 1, wherein said tubular insertfurther comprises a tubular connecting portion and means for the radialspacing of the surface of the insert from the skirt of the cup, saidtubular connecting portion being disposed between the radial spacingmeans and an end of the tubular insert adapted to interfere with thedisk-brake pad, the tubular connecting portion being intended to projectat least partially from the cup.
 10. A piston according to claim 9 inwhich the tubular connecting portion comprises a plurality ofthrough-holes for putting the hollow interior of the tubular insert intocommunication with the exterior of the piston.
 11. A disk-brakecomprising at least one piston, said piston comprising a cup having askirt and a head portion, said piston further comprising a hollowtubular insert housed in the cup, wherein a space for cooling air isdefined between an outer surface of the tubular insert and an innersurface of the skirt of the cup.
 12. A disk-brake according to claim 11in which the tubular insert comprises snap-retaining means for retainingthe insert in the cup, the tubular insert being removable from the cupafter the application of an adequate removal force.
 13. A disk-brakeaccording to claim 2 in which the snap-retaining means comprise aresilient ring housed in an annular groove formed in the outer surfaceof the tubular insert and cooperating with an annular rib on the innersurface of the cup.
 14. A disk-brake according to claim 3 in which theresilient ring is made of metal or is a metal split ring.
 15. Adisk-brake according to claim 1, in which the tubular insert comprisesmeans for the radial spacing of the surface of the insert from the skirtof the cup.
 16. A disk-brake according to claim 5 in which the radialspacing means are a ring of teeth which define ducts for the circulationof air between the space between the insert and the skirt of the cup andthe exterior of the piston.
 17. A disk-brake according to claim 1, thetubular insert projecting axially from the cup.
 18. A disk-brakeaccording to claim 1, the tubular insert comprising a plurality ofthrough-holes for putting the hollow interior of the insert intocommunication with the space between the insert and the skirt of thecup.
 19. A disk-brake according to claim 1, wherein said tubular insertfurther comprises a tubular connecting portion and means for the radialspacing of the surface of the insert from the skirt of the cup, saidtubular connecting portion being disposed between the radial spacingmeans and an end of the tubular insert adapted to interfere with thedisk-brake pad, the tubular connecting portion being intended to projectat least partially from the cup.
 20. A disk-brake according to claim 19in which the tubular connecting portion comprises a plurality ofthrough-holes for putting the hollow interior of the tubular insert intocommunication with the exterior of the piston.